1. Field of the Invention
The present invention relates to an air/fuel ratio feedback control system including an air/fuel ratio sensor which varies its output linearly in correspondence with the air/fuel ratio of an internal combustion engine so that air/fuel ratio feedback control may be executed on the basis of the detection result of that sensor by an electronic control unit composed of a microprocessor or the like. More particularly, the invention is related to an abnormality diagnosing system for such an air/fuel ratio feedback control system.
2. Description of Related Art
In an air/fuel ratio feedback control system of recent years, an air/fuel ratio sensor (e.g., a limit current type oxygen sensor) has been used to linearly detect the air/fuel ratio of an engine in accordance with the oxygen concentration in an exhaust gas. A microprocessor obtains the detection result of the air/fuel ratio by the sensor to control the fuel injection rate into the internal combustion engine. In this case, the microprocessor calculates the air/fuel ratio correction coefficient on the basis of the air/fuel ratio detection result from the air/fuel ratio sensor to correct the fuel injection rate with the air/fuel ratio correction coefficient. As a result, optimal combustion is realized in the internal combustion engine to reduce the content of noxious by-products (e.g., CO, HC, NOx and so on) in the exhaust gas.
In the air/fuel ratio feedback control system, on the other hand, as the reliability of the air/fuel ratio to be detected by the air/fuel ratio sensor decreases, the control accuracy of the system seriously deteriorates. Therefore, the prior art has desired a technique for performing an abnormality diagnosis of the air/fuel ratio sensor highly accurately. Herein, an abnormality of the air/fuel ratio sensor or a control abnormality due to the electronic control unit (e.g., the microprocessor) will be defined as an abnormality of the air/fuel ratio feedback control system.
The characteristic abnormality of the sensor output will be described with reference to FIG. 46A. In normal sensor operation, the detected air/fuel ratio (i.e., the detected .lambda.) and the real air/fuel ratio (i.e., the real .lambda.) are substantially identical, as shown by the solid line in FIG. 46A. At the abnormal sensor time, the detected .lambda. is smaller or larger than the real .lambda., as shown by broken lines. This characteristic abnormality is thought to occur from aging such as electrode aggregation or separation or element cracking (e.g., the cracking of a diffused resistance layer or a solid electrolyte layer) or from a conduction or insulation defect.
The abnormality in which the responsiveness of the sensor output drops will be described with reference to FIG. 46B. When the responsiveness of the sensor output drops, the transition of the detected .lambda. is delayed from that of the real .lambda. when the fuel injection rate changes from the lean side to the rich side, as shown. This drop in the responsiveness is thought to occur from blockage of a cover or a porous electrode layer due to the contamination of the sensor or from insufficient activity due to heater trouble.
As described above, problems with the air/fuel ratio sensor output may be caused by a number of factors, and a technique for making an abnormality diagnosis of the air/fuel ratio sensor highly accurately has been desired. To this end, a system for diagnosing the sensor abnormality on the basis of the behavior of the detected air/fuel ratio by the air/fuel ratio sensor or the behavior of the value FAF has been developed.
In the feedback control of the air/fuel ratio, generally speaking, the evaporated fuel (or evaporated gas) in the fuel tank, for example, is to be released to the engine intake system, and the value FAF may deviate erratically or at a transient running time from a target value (e.g., a reference value=1.0). Even when, moreover, a learned air/fuel ratio value is cleared immediately after the battery was exchanged, an error may occur in the value FAF in accordance with the individual difference in the engine. As a result, if the diagnosis of the existing sensor abnormality is executed at the varying time of the value FAF, there is a problem that an erroneous diagnosis is liable to occur.
Further, prior art documents such as Japanese Patent Publication Laid-Open No. 62-225943 entitled "Method of Detecting Abnormality of Oxygen Concentration Sensor", disclose an abnormality diagnosing procedure for detecting an abnormality of a connection system in the limit current type oxygen concentration sensor in accordance with an applied voltage and a detected current.
The prior art described above can detect an abnormality in the circuit construction such as a breakage or short-circuit of the connection system. However, when the air/fuel ratio sensor degrades, the symptom of a resultant abnormality cannot be detected. In other words, it is impossible to determine the accuracy of the air/fuel ratio detected by the air/fuel ratio sensor (whether or not the sensor output is normal), that is, to determine the reliability of the sensor output. Further, it is not possible to diagnose the reliability of the electronic control system of the air/fuel ratio feedback by using the detection result of the air/fuel ratio sensor.